I've been on an early science fiction kick of late. Jules Verne, HG Wells, that sort of thing. The stuff that created the genres I grew up with, but never actually got round to reading. I'm finding it fascinating from a sociological and history of science viewpoint (the conceived propulsion system of the Nautilus says an awful lot about the state of electrical engineering in the 1860s, for example, and the book in general is practically a homage to the Victorian classification-is-all approach to biology).
In that vein, how about M. P. Shiel's The Purple Mist. One of the defining works of the post-apocalyptic genre.
Top tip: Older hard SF is always best appreciated with googlepedia on standby to translate the wacky units and archaic chemical names into something that makes sense.